Inconsistent Edge Length in Blender 2.9
I can't be sure this is your issue, but overlay 'Edge Lengths' are displayed in Object space: that is, before any Object-level transform the object is carrying.
This plane has been scaled by 2 in Edit mode, doubling its vertex X-locations in Object space. The rectangle has then been scaled by 0.5 in X, in Object mode, restoring it to a square in World Space, but only by virtue of the transform the object is carrying.

If you CtrlA 'Apply' the scale-transform of the object, that will reset the object's X-scale to 1, and bake the object's scale into the Object-space locations of its vertices.
(The shipped add-on MeasureIt displays dimensions in World space.)